A common technique for making microelectronic circuit connections between the die of an integrated circuit device and the pins of that device is wire ball bonding. The die typically has one or more bond pads electrically coupled to the microelectronic circuitry. These bond pads are often comprised of aluminum or an aluminum alloy. Each bond pad has a wire, often a gold wire, bonded to it and a corresponding lead finger. A lead finger is a conductor that serves as, or is connected to, one of the leads of the integrated circuit device. The bond formed between the wire and the bond pad creates an intermetallic structure. The strength of the intermetallic structure depends upon a number of factors, including the temperature at which the bond is formed. Higher temperatures tend to improve the extent of intermetallic coverage. The highest bond strength is obtained by achieving maximum intermetallic coverage.
One of the most widely used processes for microelectronic bonding is thermosonic bonding. In a thermosonic bonding process, the die of the integrated circuit and, hence, the bond pads are heated to a relatively high temperature. Such heating is normally accomplished using a heater block. A capillary holds a wire with a ball formed at the end. This capillary is lowered so that the ball contacts the bond pad. At this point, the capillary applies mashing force to the ball and also supplies ultrasonic energy to form the bond. The other end of the wire is then stitch bonded to the appropriate lead finger on the lead frame.
Normally, the die must be heated to a high temperature, sometimes as high as between 240.degree. C. and 290.degree. C. Some modern devices, such as micromechanical devices cannot, without damage, be heated to temperatures beyond approximately 80.degree. C. during the bonding process. For such devices, microelectronic bonding necessarily must occur at lower temperatures. Heat is an important variable in intermetallic formation, however, and it is more difficult to achieve adequate intermetallic bonding at lower temperatures.
Heating the lead frame of an integrated circuit during the wire ball bonding process can create a number of problems. Heat can cause deformation of the lead frame, causing stress in one or more lead fingers. Heat can also cause two or more lead fingers to bend closer to one another. Such bending sometimes causes lead fingers to be bonded together when a stitch bond is made. This phenomenon is particularly problematic with modern VLSI circuits that have a high pin count. Also, because the lead fingers are heated when the wire is attached during the bonding process, subsequent cooling of the lead finger may cause it to bend in a direction such that the wire is under tension. Tension due to lead finger bending may decrease the longevity of the bond.
Texas Instruments Incorporated has developed a high frequency ultrasonic bonding process that can achieve good intermetallic bonding at lower temperatures. That process is described, for example, in U.S. Pat. No. 5,244,140, entitled "Ultrasonic Bonding Process Beyond 125 KHz." This ultrasonic bonding process, however, causes the capillary to wear out faster due to the higher frequency ultrasonic energy needed to form the bond.
A process is needed that allows adequate intermetallic formation using lower frequency thermosonic bonding without the need for heating the die and lead frame to high temperatures.